Storage stable heat curable organosiloxane compositions containing a microencapsulated catalyst and method for preparing said catalyst

ABSTRACT

The present invention provides microencapsulated hydrosilylation catalysts for one-part heat curable organosiloxane compositions. Compositions containing these exhibit long-term storage stability yet cure rapidly when heated above the melting point of the encapsulating polymer. At least a portion of these microcapsules are less than one micron in diameter, and substantially all are less than about three microns in diameter. Compositions containing the preferred platinum group metals as catalysts are optically transparent. 
     The microencapsulated curing catalysts of this invention are prepared by irradiating with UV light in the wavelength range of from 300 to 400 nanometers a solution containing (1) at least one of a specified group of photocrosslinkable organosiloxane compounds derived from propargyl esters of carboxylic acids containing a terminal aromatic hydrocarbon radical and at least two ethylenically unsaturated carbon atoms and (2) a liquid or solubilized hydrosilylation catalyst.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to curable organosiloxane compositions. Moreparticularly, this invention relates to one-part organosiloxanecompositions that cure by means of a hydrosilylation reaction andcontain a novel microencapsulated curing catalyst. The compositionsexhibit long term storage stability under ambient conditions yet curerapidly at elevated temperatures.

2. Description of the Prior Art

Organosiloxane compositions cure by a variety of reactions. Some of themore common curing means involve 1) a free radical reaction initiated bythe heat-induced decomposition of an organic peroxide or thedecomposition of a photoinitiator in the presence of ultraviolet lightand 2) the reaction between a hydroxyl-containing polyorganosiloxane andan organosilicon compound containing two or more hydrolyzable groupsthat occurs under ambient conditions in the presence of moisture and asuitable catalyst.

One of the more useful classes of polyorganosiloxane compositions curesby a reaction between silicon-bonded hydrogen atoms and either siliconbonded alkenyl radicals or hydroxyl groups. These reactions arecatalyzed by metals from the platinum group of the periodic table orcompounds of these metals. The advantages of these compositions includetheir rapid curing rate, particularly at elevated temperatures, theabsence of objectionable by-products produced during the curing ofcompositions containing organic peroxides or silanes with hydrolyzablegroups such as acetoxy or methoxy, and the difficulty of achievingcomplete curing of moisture-curable organosiloxane compositions appliedin thick layers.

Compositions that cure by a hydrosilylation reaction typically contain apolydiorganosiloxane with at least two ethylenically unsaturatedhydrocarbon radical per molecule, an organohydrogensiloxane containingat least two silicon bonded hydrogen atoms per molecule in an amountsufficient to achieve curing of the composition and a platinum-orrhodium-containing catalyst in an amount sufficient to promote curing ofthe composition. Fillers and other additives may be present for thepurpose of modifying physical and/or chemical properties of thecomposition either prior to or following curing.

Because organosiloxane compositions that cure by a platinum-catalyzedhydrosilylation reaction begin to cure even at ambient temperature oncethe reactants are combined, the catalyst and the organohydrogensiloxanereactant are usually packaged in separate containers and are notcombined until it is desired to cure the composition. Even if thecomposition contains one or more of the known platinum catalystinhibitors it cannot be stored in a single container for more than a fewhours.

One of the alternatives proposed in the prior art to supplyingplatinum-catalyzed curable organosiloxane compositions as two-packagematerials is to isolate either the catalyst or theorganohydrogensiloxane within a matrix of a material that is solid underthe conditions encountered during storage of the curable composition,yet allows the entrapped reactant or catalyst to escape and mix with theother ingredients when it is desired to cure the composition.

The prior art discloses a number of different one-part curableorganosiloxane compositions containing microencapsulated reactants orcatalysts. An example of this type of composition is described in U.S.Pat. No. 4,528,354, which issued to McDougal and Dougherty on Jul. 9,1985. This patent describes one-part peroxide curable silicone rubbercompositions. The compositions include a microencapsulated liquid phasecontaining an organic peroxide in a shell of a resinous thermosettingmaterial that is impervious to the peroxide.

The capsules are designed to rupture under a given internal vaporpressure that is generated by the encapsulated liquid when the curablecomposition containing the microcapsules is heated.

Because release of the peroxide is dependent upon rupturing rather thanmelting of the shell separating the peroxide from the other ingredientsof the organosiloxane composition, the composition and thickness of theshell must be carefully controlled to ensure that the rupture of thecapsules will occur reproducibly within the temperature range used tocure the organosiloxane composition.

U.S. Pat. No. 4,604,444, which issued to Donnadieu on Aug. 5, 1986describes storage stable polyorganosiloxane compositions comprising apolyhydroxylated polyorganosiloxane, a polyfunctional acyloxysilane anda microencapsulated accelerator that either contains or generates water.The encapsulated material can be released using heat and/or irradiation.Suitable encapsulating materials include polystyrene,acrylonitrile-styrene copolymers, and poly(methyl methacrylate). Thispatent does not suggest using microencapsulated materials inorganosiloxane compositions curable by means other than the reaction ofpolyhydroxylated polyorganosiloxanes with acyloxysilanes.

U.S. Pat. No. 4,461,854, which issued to Smith on Jul. 2, 1984 teachestwo-part curable organosiloxane compositions. One part contains asilanol-terminated polyorganosiloxane and the second component containsa curing agent, a filler and an encapsulated catalyst. The catalyst is aspecified group of metal salts of carboxylic acids where the metal is,for example, tin, lead or zirconium The encapsulating material ispreferably a salt of a carboxylic acid that does not promote roomtemperature curing of the composition. The encapsulated catalystprolongs the bath life of the curable composition.

U.S. Pat. No. 4,481,341, which issued to Schlak et al. on Nov. 6, 1984and Japanese published application No. 49/134,786, published on Dec. 25,1974 describe thermosetting organosiloxane compositions comprising apolyorganosiloxane containing at least two ethylenically unsaturatedhydrocarbon radicals per molecule, a polyorganohydrogensiloxanecontaining at least two silicon bonded hydrogen atoms per molecule and aplatinum-containing catalyst that is dispersed in a finely divided,solid matrix, such as a silicone resin or an organic resin. Theconcentration of catalyst is from 0.001 to 5 percent by weight ofplatinum metal.

The finely divided material in which the catalyst is dispersed isvirtually insoluble in either the aforementioned polyorganosiloxane orpolyorganohydrogensiloxane and melts or softens at a temperature between70 and 250 degrees C. The alleged advantage of the compositionsdisclosed in the patent to Schlak et al. is that the catalyst remainsisolated from the other ingredients of the curable composition until thecomposition is heated sufficiently to melt the material in which thecatalyst is dispersed. Because the organosilicon compounds present inthe composition will not cure in the absence of the catalyst, thecomposition can allegedly be stored for long periods of time withoutundergoing curing or even an increase in viscosity.

A disadvantage of the curable organosiloxane compositions described bySchlak et al. and the published Japanese patent application is themethod taught to prepare the catalyst/resin composition. A solid blockor sheet of resin containing the platinum composition dispersedthroughout is ground to a fine powder. Based on the random nature of thegrinding operation there is a reasonable probability that some of theparticles will contain platinum catalyst on their surface. Even traceamount of platinum have been shown to cause premature curing of the typeof organosiloxane composition exemplified in this patent.

One way to avoid the inherent disadvantages of the catalyst compositionsdescribed in the Schlak et al. patent, is to completely microencapsulatefinely divided particles or droplets of a catalyst composition within amaterial that is impermeable to the catalyst and effectively isolates itfrom the reactive ingredients of a curable organosiloxane composition.The encapsulant melts or softens at the desired curing temperature ofthe composition. A variety of methods for microencapsulating materialsare known in the art.

U.S. Pat. No. 4,874,667, which issued on Oct. 17, 1989 to Lee et al. andis assigned to the same party as the present application disclosesone-part organosiloxane compositions that cure by a platinum-catalyzedhydrosilylation reaction. The platinum catalyst is microencapsulated inone or two layers of a thermoplastic organic polymer. The diameter ofthe microencapsulated catalyst particles are less then 100 microns.

A disadvantage of the preparative methods described in the Lee et al.patent and related U.S. Pat. No. 4,766,176, issued on Aug. 23, 1988 andU.S. Pat. No. 4,784,879. issued on Nov. 15, 1988, is the inability ofthese methods to yield microcapsules of sufficiently small size thatcurable compositions containing these microcapsules are opticallytransparent. Curable compositions containing these microcapsules aretranslucent or opaque.

U.S. Pat. No. 5,066,699, which issued on Nov. 19, 1991 describes amethod for preparing a microencapsulated platinum-containinghydrosilylation catalyst. A solubilized hydroxyl-containingethylenically unsaturated organic compound is photopolymerized in thepresence of the catalyst, a photoinitiator for the polymerization and anoptional surfactant. Substantially none of the microcapsules is largerthan 3 micrometers in diameter. This patent also discloses storagestable one-part curable organosiloxane compositions containing amicroencapsulated catalyst prepared as described in the patent. Thecompositions are cured by heating them at a temperature that is at leastequal to the softening temperature of the polymer portion of themicroencapsulated catalyst.

One objective of this invention is to provide a novel type ofmicroencapsulated platinum group metal-containing curing catalyst thatdoes not detract from the optical transparency or the cure rate atelevated temperatures of curable organosiloxane compositions containingthe catalyst.

A second objective is to provide a novel method for preparingmicroencapsulated hydrosilylation reaction catalysts that are effectivecuring catalysts for organosiloxane compositions.

Another objective is to provide optically transparent one-part storagestable organosiloxane compositions containing the microencapsulatedcuring catalysts of this invention. The compositions can be cured eitherby heating them or exposing them to ultraviolet radiation in the rangeof from 200 to 260 nanometers (nm).

SUMMARY OF THE INVENTION

The present invention provides microencapsulated hydrosilylationcatalysts for one-part organosiloxane compositions that cure by ahydrosilylation reaction. In addition to exhibiting long-term storagestability compositions containing the preferred platinum group metalcatalysts are optically transparent in the absence of opacifyingadditives. At least a portion of these microcapsules are less than onemicron in diameter, and substantially all are less than about threemicrons in diameter.

The microencapsulated curing catalyst ingredient of the present curablecompositions is prepared by the photoinitiated crosslinking oforganosilicon derivatives prepared by a hydrosilylation reaction betweenan organohydrogensiloxane and a propargyl ester wherein the carboxylicacid portion contains a sequence of at least two conjugated ethylenicdouble bonds between the carbopropynoxy group and a terminal aryl oralkaryl radical.

DETAILED DESCRIPTION OF THE INVENTION

This invention provides a microencapsulated catalyst compositioncomprising a liquid or solubilized hydrosilylation catalyst encapsulatedwithin at least one layer of at least one organosiloxane compound, wheresaid organosiloxane compound is the cross-linked product obtained byexposing an organosiloxane compound containing repeating units of theformula

    QR.sup.1.sub.a SiO.sub.(3-a)/2

to ultraviolet light of a wavelength from 300 to 400 nm, where Qrepresents the radical

    R.sup.2 (CH═CH).sub.n --CH═CH--CH═CR.sup.3 C(O)OCH.sub.2 CH═CH--,                                              (1)

each R¹ is individually selected from the group consisting ofunsubstituted and substituted monovalent hydrocarbon radicals, R²represents an aryl, alkoxyaryl or alkaryl radical, R³ is --C.tbd.N or--C(O)OR⁴, where R⁴ represent an unsubstituted monovalent hydrocarbonradical, a is 0, 1 or 2 and n is 0 or a positive integer, with theproviso that n is 0 only when R² is naphthyl.

Unsubstituted hydrocarbon radicals that can be represented by R¹ and R⁴include but are not limited to alkyl radicals containing from 1 to 10carbon atoms, such as methyl, ethyl and propyl, cycloalkyl radicals suchas cyclohexyl, aryl such as phenyl, alkaryl such as tolyl and xylyl andaralkyl such as benzyl. Substituents that can be present on R¹ includebut are not limited to halogens such a chlorine, bromine and fluorine.Most preferably R¹ and R⁴ are methyl or phenyl and R¹ is3,3,3-trifluoropropyl. The preferences for R¹ are based on theavailability of the corresponding chlorosilanes used to prepare theorganohydrogensiloxanes used to prepare the present organosiloxanecompounds.

Preferred hydrosilylation catalysts are compounds of metals from theplatinum group of the periodic table.

This invention also provides a one-part, storage stable, heat curableorganosiloxane composition comprising

A. a liquid or gum polyorganosiloxane containing an average of at leasttwo ethylenically unsaturated hydrocarbon radicals per molecule;

B. as a curing agent for said composition, an organohydrogensiloxanecontaining an average of at least two silicon-bonded hydrogen atoms permolecule, and

C. an amount of a microencapsulated hydrosilylation catalyst sufficientto promote curing of said composition at temperatures above 100° C. orin the presence of ultraviolet radiation having a wavelength in therange from 200 to 260 nanometers.

where the concentration of said curing agent is sufficient to cure saidcomposition, the sum of the average number of ethylenically unsaturatedhydrocarbon radicals per molecule of polyorganosiloxane (A) and theaverage number of silicon-bonded hydrogen atoms in each molecule of saidcuring agent (B) is greater than 4, said catalyst is microencapsulatedwithin a matrix or layer of at least one crosslinked organosiloxanecompound. The non-crosslinked organosiloxane compound component used tomake the crosslinked organosiloxane comprises at least two units of theformula

    R.sup.2 (CH═CH).sub.n --CH═CR.sup.3 C(O)OCH.sub.2 CH═CH--SiR.sup.1.sub.a O.sub.(3-a/2)                  (1)

where each R¹ is individually selected from the group consisting ofunsubstituted and substituted monovalent hydrocarbon radicals, R²represents an aryl, alkoxyaryl or alkaryl radical, R³ is --C.tbd.N or--C(O)OR⁴ where R⁴ is hydrogen or an unsubstituted monovalenthydrocarbon radical, a is 0, 1 or 2 and n is 0 or a positive integer,with the proviso that n can be 0 only when R² represents a naphthylradical, and said organosiloxane compound has been crosslinked byexposure to ultraviolet radiation in the wavelength range of from 300 to400 nm.

A characteristic feature of curable organosiloxane compositionscontaining the microencapsulated hydrosilylation catalysts, alsoreferred to herein as microcapsules, of this invention is their opticaltransparency. The transparency is believed due to the sub-microndiameter of at least a major portion, typically at least 50 percent, ofthe microcapsules and the low concentration of microcapsules required toachieved a rapid cure rate at elevated temperatures. Substantially noneof the present microcapsules are larger than about 3 microns indiameter.

The present invention also provides a method for preparing themicroencapsulated hydrosilylation catalysts. This method comprises thefollowing sequence of steps:

1) exposing a solution consisting essentially of at least onephotocrosslinkable organosiloxane compound, a solubilized platinumcontaining hydrosilylation catalyst and a volatile solvent toultraviolet radiation within the wavelength range of from 300 to 400nanometers for a sufficient time to crosslink said compound andevaporate substantially all of said solvent, and

2) isolating said microencapsulated catalyst,

where the repeating units of said organosiloxane compound exhibit theformula

    R.sup.2 (CH═CH).sub.n --CH═CR.sup.3 C(O)OCH.sub.2 CH═CH--SiR.sup.1.sub.a O.sub.(3-a/2)

where each R¹ is individually selected from the group consisting ofunsubstituted and substituted monovalent hydrocarbon radicals, R²represents an aryl, alkoxyaryl or alkaryl radical, R³ is --C.tbd.N or--C(O)OR⁴ where R⁴ is hydrogen or an unsubstituted monovalenthydrocarbon radical, a is 0, 1 or 2 and n is or a positive integer, withthe proviso that n can be 0 only when R² represents a naphthyl radical.

The hydrosilylation catalyst is effectively isolated from the otheringredients of the curable organosiloxane composition until thecomposition is either heated to the melting or softening point of thecrosslinked organosiloxane compound that encapsulates thehydrosilylation catalyst or the crosslinked organosiloxane compound isconverted to an uncrosslinked material by exposing the curablecomposition to ultraviolet radiation in the range from 200 to 260nanometers (nm), referred to as the "far" or "deep" ultraviolet range.

The ability of organosiloxane compounds derived from esters ofalpha-cyano-beta-styrylacrylic acid to undergo a reversiblephotoinitiated crosslinking reaction is reported by R. Mercier et al. inthe European Polymer Journal, 24, 7 (639-645 (1988).

The novelty of the present catalyst compositions resides in thecrosslinked organosiloxane compound or compounds that encapsulate thehydrosilylation catalyst. The compounds are the product of aphotointiated crosslinking involving organosiloxane compounds preparedby reacting an organohydrogenpolysiloxane or organohydrogendisiloxanewith the propargyl radical of a propargyl ester of a carboxylic acidcontaining a sequence of at least two conjugated ethylenic double bondsbetween the carboxyl group of the ester and a terminal aryl, alkoxyarylor alkaryl radical on the acid portion of the ester. When R³ of formula1 represents a naphthyl radical the sequence of conjugated double bondscan be replaced by a single ethylenic double bond.

The organosiloxane compounds represented by formula 1 are considerednovel and are claimed in a copending patent application filedconcurrently with the present specification.

The photo-initiated crosslinking of polyorganosiloxanes obtained byreacting an organohydrogensiloxane with the reaction product of avinylchlorosilane and an alkali metal salt of cinnamic, beta-(2-furyl)acrylic or alpha-cyano-beta-styryl acrylic also referred to as2-cyano-5-phenyl-2-4-pentadienecarboxylic) acids is described by R.Mercier, X. Coqueret and coworkers in the European Polymer Journal, vol.24, No. 7, pages 639-645 (1988). The polyorganosiloxanes arecharacterized by the group ##STR1## where the group ##STR2## is derivedfrom the initial organohydrogensiloxane reactant.

The organosiloxane compounds used to prepare the encapsulating materialsfor the present microencapsulated hydrosilylation catalysts differ fromthe polyorganosiloxanes described by Mercier and coworkers in thepresence of the group ##STR3## is a residue of theorganohydrogensiloxane that is reacted with a propargyl esterrepresented by formula 2.

    R.sup.2 (CH═CH).sub.n --CH═CR.sup.3 C(O)OCH.sub.2 CH.tbd.CH(2)

The advantage of using this propargyl ester in place of the alkali metalacid salts of Mercier et al. is the unexpected selectivity of anorganohydrogensiloxane reactant for the propargyl group in preference tothe conjugation series of carbon double bonds in the acid portion of theester.

The present organosiloxane compounds undergo the same type of reversiblephotoinitiated crosslinking reaction described in the aforementionedarticle by Mercier and coworkers. The present inventors discovered thatthis reaction can be utilized to microencapsulate liquid and solubilizedhydrosilylation catalysts. These catalysts typically contain a metalfrom the platinum group of the periodic table, which includes platinum,palladium and rhodium. The catalyst ca be released by exposing themicroencapsulated catalyst to ultraviolet radiation in the wavelengthrange of from 200 to 260 nm or by heating the microencapsulated catalystto temperatures above about 120° C.

The photoinitiated cross-linking reaction of the present organosiloxanecompounds is believed to involve formation of cyclobutane rings by pairsof ethylenically unsaturated carbon atoms on adjacent molecules of theorganosiloxane. This reaction can be depicted as ##STR4##

A major portion (at least ˜80%) of the intermolecular bonds formedduring this reaction can be broken by exposing the reaction product toultraviolet radiation in the range from 200 to 260 nm, thereby at leastpartially converting the intermolecularly bonded compound to theoriginal organosiloxane units represented by formula 1.

The microencapsulated hydrosilylation catalysts of the present inventioncan be prepared by irradiating a mixture of the catalyst in liquid orsolubilized form and an organosiloxane compound containing unitsrepresented by formula 1 with ultraviolet radiation in the wavelengthrange from 300 to 400 nanometers (nm). This wavelength range is alsoreferred to as the "near UV range". A wavelength of about 360 nanometersis preferred, based on the availability of UV sources and filtersexhibiting emission in this wavelength region.

The amount of solvent used should be the minimum required to dissolvethe hydrosilylation catalyst and the organosiloxane compound. An excessof solvent has been shown to decrease the rate of crosslinking of theorganosiloxane compound in the presence of ultraviolet radiation.

The duration of the exposure required to achieve crosslinking of theorganosiloxane compound and encapsulation of the hydrosilylationcatalyst is a function of a number of variables, including but notlimited to the intensity of the UV radiation reaching the organosiloxanecompound and the energy threshold of the crosslinking reaction.

Preparation of the Photopolymerizable Organosiloxane Compound

The present organosiloxane compounds are prepared by reacting anorganohydrogensiloxane with a propargyl ester of a carboxylic acidcontaining a sequence of conjugated carbon-to-carbon double bonds. Thepropargyl ester exhibits the formula

    R.sup.2 (CH═CH).sub.n --CH═CR.sup.3 C(O)OCH.sub.2 C.tbd.CH(2)

In this formula R² represents an aryl, alkoxyaryl or alkaryl radical, R³is --C.tbd.N or --C(O)OR⁴ where R⁴ is hydrogen or an unsubstitutedmonovalent hydrocarbon radical, and n is 0 or a positive integer, withthe proviso that n can be 0 only when R² represents a naphthyl radical.

Preparation of the Propargyl Ester (Formula 2)

A preferred class of the propargyl esters represented by formula 2 canbe prepared using the Knoevenagel reaction, a base-catalyzedcondensation between an aldehyde and the ester of an acid containing alabile hydrogen atom in the alpha position relative to the carbonylgroup. This reaction is typically catalyzed by a base, and can beillustrated by the general equation

    R'CHO+R''CH.sub.2 COOR'''→R'CH═C(R'')COOR'''

where R', R" and R"' represent monovalent hydrocarbon radicals.

The reaction used to prepare the propargyl ester of formula 2 can berepresented by the equation ##STR5##

The reactants used to prepare this propargyl ester are the propargylester of a carboxylic acid containing a labile hydrogen atom on thealpha carbon relative to the carbonyl group (formula 4), referred tohereinafter as the propargyl ester reactant, and an aldehyde containinga terminal aromatic hydrocarbon radical represented by R² (formula 3).The number of ethylenic double bonds in the aldehyde is represented byn, the value of which is preferably 0 or 1, with the proviso that R³must represent naphthyl when n is 0.

The Ethylenically Unsaturated Aldehyde Reactant (Formula 3)

The ethylenically unsaturated aldehyde used in the Knoevenagel reactioncan be represented by formula 3

    R.sup.2 (CH═CH).sub.n C(O)H                            (3)

where R² represents an aryl radical such as phenyl or naphthyl, analkoxyaryl radical or an alkaryl radical, and the value of n is 0 or apositive integer, with the proviso that n can be 0 only when R²represents a naphthyl radical.

Suitable aldehydes represented by formula 3 contain an aromatichydrocarbon radical at the terminal position and include but are notlimited to cinnamaldehyde and products of an aldol condensation reactionbetween cinnamaldehyde, o- or p-methoxy cinnamaldehyde, benzaldehyde,naphthaldehyde or other aldehyde containing an aromatic hydrocarbonradical at the terminal position and an ethylenically unsaturatedaliphatic aldehyde such as crotonaldehyde or acrolein.

When the carbonyl group of one of the aldehydes is bonded to an aromatichydrocarbon ring structure or separated from the ring structure by a--CH═CH-- group as in cinnamaldehyde, typically only one condensationproduct is formed.

The ethylenically unsaturated carbon atoms of the unsaturated aliphaticaldehyde 6 are adjacent to the aldehyde group [--C(O)H]. If bothaldehydes (5 and 6) used in the aldol condensation contain ethylenicallyunsaturated carbon atoms in this position, this will maximize the numberof conjugated double bonds present in the reaction product.

The aldol condensation reaction between cinnamaldehyde (5a) andcrotonaldehyde (6a) can be represented by the following equation, wherePh represents a phenyl radical. ##STR6##

If desired it should be possible to repeat the condensation reaction toincrease the number of sequential conjugated double bonds in theethylenically unsaturated aldehyde that is subsequently reacted with theester reactant represented by formula 5 to prepare the propargyl estersof the present invention represented by formula 2.

The Propargyl Ester Reactant (Formula 4)

The propargyl ester reactant used in the Knoevenagel reaction contains alabile hydrogen atom on the carbon atom adjacent to the carboxyl groupand can be represented by formula 4.

    R.sup.3 H.sub.2 CC(O)OC.tbd.CH                             (4)

where R³ is as previously defined. R³ preferably represents a cyanoradical, this preference being based on the availability ofalpha-cyanoacetic acid.

The Knoevenagel condensation reaction between the propargyl esterreactant (4) and the aromatic aldehyde (3) to form the propargyl esterrepresented by formula 2 is typically conducted at ambient temperatureor below in the presence of a catalytic amount of an amine. Suitableamine catalysts include but are not limited to aliphatic amines,aromatic amines such as aniline and p-nitroaniline. Heterocyclic aminessuch as piperidine are preferred.

The condensation reaction is typically carried out with the reactantsdissolved in a common solvent, particularly when the reaction product isa solid. Useful solvents include but are not limited to cyclic etherssuch as tetrahydrofuran and dioxane. The present inventors discoveredthat mixtures of cinnamaldehyde and naphthaldehyde containing from about30 to about 50 weight percent of naphthaldehyde are preferred for use inpreparing the organosiloxanes represented by formula 1, based on thelong shelf life and rapid cure exhibited by organosiloxane compositionscontaining hydrosilylation catalysts encapsulated in the crosslinkedmixture of siloxanes prepared using this combination of aldehydes.

Preparation of the Organosiloxane Compound, a Silylated Propargyl Ester(Formula 1)

The organosiloxane compounds that are photopolymerized in the presenceof a hydrosilylation catalyst to prepare the microencapsulated catalystsof this invention can be prepared by the reaction of anorganohydrogensiloxane with the propargyl group of the ester prepared asdescribed in the preceding section of this specification and representedby formula 2.

    R.sup.3 (CH═CH).sub.n --CH═CR.sup.2 C(O)OCH.sub.2 C.tbd.CH(2)

The reaction between silicon-bonded hydrogen atoms and a carbon-carbondouble bond or triple bond is referred to as a hydrosilylation reactionand is typically catalyzed by a metal from the platinum group of theperiodic table or a compound of such a metal. In addition to platinumthe platinum group of metals includes rhodium and palladium. To providethe desired selectivity of reaction at the actylenic carbon atoms of thepropargyl ester it is preferred to use platinum or a compound of thismetal.

Chloroplatinic acid, and more particularly complexes of chloroplatinicacid with liquid ethylenically unsaturated organosilicon compounds suchas sym-tetramethyldivinyldisiloxane, are preferred catalysts for thereaction of the propargyl ester with the organosilicon compoundcontaining silicon-bonded hydrogen atoms. This preference is based onthe selectivity of these catalysts in limiting the site of thehydrosilylation reaction to the propargyl group of the ester representedby formula 3 and the high yield of the desired organosilicon compound.

The propargyl ester represented by formula 2 and theorganohydrogensiloxane are preferably used in substantially equimolaramounts to avoid undesirable side reactions between the silicon bondedhydrogen atoms and the conjugated carbon-carbon double bonds present inthe acid portion of the propargyl ester.

The hydrosilylation reaction involving the propargyl ester is generallycarried out in the presence of a solvent that will not participate inthis reaction. Preferred solvents include but are not limited to liquidaromatic and saturated aliphatic hydrocarbons.

While the hydrosilylation reaction will proceed at room temperature, thereaction mixture is preferably heated at between 70° and 110° C. toincrease the rate of the reaction. The preferred temperature will dependupon a number of variables, including the type of organohydrogensilaneor organohydrogensiloxane used. The course of the reaction canconveniently be followed using infrared spectroscopy to observe thedecreasing concentration of the propargyl (--H₂ C--C.tbd.CH) and SiHgroups as the reaction proceeds.

Preferred organohydrogensiloxanes include but are not limited tosymmetrical tetraalkyldihydrogendisiloxanes, resinous organosiloxanecopolymers containing phenylsilsesquioxane (PhSiO_(3/2)) anddimethylhydrogensiloxy units, and substantially linearorganopolysiloxanes wherein at least a portion of the repeating unitsare represented by the formula R¹ HSiO, where R¹ represents anunsubstituted or substituted monovalent hydrocarbon radical as definedin the preceding specification, any remaining non-terminal units are R¹₂ SiO, and the terminal units are triorganosiloxy ordiorganohydrogensiloxy, where the organic groups are hydrocarbonradicals selected from the same group as R¹. Alternatively, thesilicon-bonded hydrogen atoms can be present only at the terminalpositions of the organopolysiloxane molecule.

The organosiloxanes obtained as products of the hydrosilylation reactionare typically solids at room temperature and can be purified usingconventional recrystallization techniques.

The Hydrosilylation Catalyst

Platinum-containing hydrosilylation catalysts suitable for preparing themicroencapsulated catalysts of this invention include reaction productsof chloroplatinic acid with an ethylenically unsaturated organosiliconcompound such as sym-divinyltetramethyldisiloxane. These reactionproducts are described by Willing in U.S. Pat. No. 3,419,593 andincorporated herein by reference. Other platinum-containinghydrosilylation catalysts considered useful for preparing the presentmicroencapsulated catalysts include those described in U.S. Pat. Nos.3,159,601; 3,159,602; 3,220,972; 3,296,291; 3,516,946; 3,814,730; and3,928,629, all of which are incorporated herein by reference asdisclosures of platinum-containing hydrosilylation catalysts.

Platinum-containing hydrosilylation catalysts form coordinationcomplexes with ethylenically or acetylenically unsaturated silicon ororganic compounds. These compounds include organosilicon compounds suchas the tetramethyldivinyldisiloxane, present as the coordinating agentin preferred platinum-containing hydrosilylation catalysts, and many ofthe inhibitors used to retard the activity of the catalyst attemperatures below about 50 degrees C.

Ethylenically or acetylenically unsaturated organic compounds containingone or more polar groups such as carbonyl or hydroxyl are particularlypreferred coordinating agents for platinum-containing hydrosilylationcatalysts.

A particularly preferred class of hydrosilylation catalysts that aremicroencapsulated by organosiloxane compounds represented by formula 1include coordination complexes of platinum. Coordinated platinumcompounds are most preferably derived from halide compounds of platinumgroup metals such as chloroplatinic acid. The chloroplatinic acid can beinitially present as the commercially available hexahydrate or in theanhydrous form disclosed by Speier in U.S. Pat. No. 2,823,218.

Polymerization of the Organosiloxane Compound of Formula 1 andPreparation of Microencapsulated Hydrosilylation Catalysts

The microencapsulated hydrosilylation catalysts of this invention areformed by exposing homogeneous solutions consisting essentially of (a)at least one of the organosiloxane compounds containing repeating unitsrepresented by formula 1 (b) a hydrosilylation catalyst and (c) asuitable volatile solvent for (a) and (b) to ultraviolet (UV) lightexhibiting a wavelength of from 300 to 400 nm. for a period of timesufficient to form microcapsules of crosslinked containing thehydrosilylation catalyst. Suitable solvents exhibit a substantial vaporpressure under ambient conditions to facilitate their removal during thephotoinitiated crosslinking reaction, and include but are not limited toketones such as acetone containing a total of from 3 to 5 carbon atomsand liquid chlorinated hydrocarbons such as methylene chloride.

In accordance with a preferred method for preparing themicroencapsulated hydrosilylation catalyst, the solution containing atleast one of the organosiloxane compounds of formula 1 and thehydrosilylation catalyst is placed in a tray or other suitable containerto form a layer of from about 2 to about 5 mm in depth. The layer ofliquid is then exposed to the radiation from a suitable lamp or otherultraviolet source exhibiting an emission maximum in the range from 300to 400 nm, preferably about 360 nm. The exposure time required to form acrosslinked organosiloxane compound is dependent upon a number ofvariables, including the intensity of the radiation at the surface ofthe liquid layer and the thickness of the layer. For intensities in therange from about 5 to about 15 milliwatts per cm² and a layer thicknessof about 4 mm the exposure time is typically from one to about threehours. Substantially all of the solvent used to dissolve theorganosiloxane compound and the hydrosilylation catalyst evaporatesduring this exposure period. It is therefore desirable to use means forremoving the volatilized solvent from the exposure area.

For some applications it may be desirable to use mixtures of two or moreorganosiloxane compounds to prepare the microencapsulatedhydrosilylation catalyst. A particularly preferred combination containsfrom 50 to 95 percent, based on the weight of said combination, of afirst organosiloxane compound containing repeating units of formula 1where R² is phenyl and n is 1. In the second organosiloxane compound R²is naphthyl and n is 0. In both compounds R³ is the cyano group, R¹ ismethyl, a is 2 and the organosiloxane compound is a disiloxane.

The product of the UV initiated polymerization is typically a solidconsisting essentially of microcapsules wherein the hydrosilylationcatalyst is completely enveloped within a layer or matrix of thecrosslinked organosiloxane compound.

The average diameter of microcapsules prepared using the method of thisinvention is less than three microns. Preferably the diameter of atleast a portion of the microcapsules is less than one micron.

To avoid premature rupturing of the microcapsules during storage thecuring catalyst should not have a significant vapor pressure attemperatures from 25 to about 60 degrees C.

For the present microencapsulated hydrosilylation catalysts, alsoreferred to as microcapsules, to function effectively in a curableorganosiloxane composition the crosslinked organosiloxane compound thatencapsulates the catalyst must melt or soften sufficiently to releasethe catalyst at or slightly below the desired curing temperature of theorganosiloxane composition.

The present microcapsules are essentially spherical in contour withdiameters of up to about 3 microns. Diameters of less than 3 microns arepreferred to ensure that curable compositions containing thesemicrocapsules will be optically transparent.

The present inventors found it difficult to encapsulate all of thehydrosilylation catalyst in a microcapsule containing one layer ofcrosslinked organosiloxane material when the hydrosilylation catalystexceeds about 5 percent of the combined weight of the catalyst and thecrosslinked organosiloxane compound present as the shell portion of themicrocapsules. The catalyst preferably constitutes from about 2 to about2.5 weight percent of the microcapsules.

Curable Organosiloxane Compositions Containing the presentMicroencapsulated Catalyst

In addition to the microencapsulated curing catalyst described in thepreceding sections of this specification the curable organosiloxanecompositions of this invention typically comprise a polyorganosiloxane,referred to hereinafter as ingredient A, containing an average of atleast two ethylenically unsaturated hydrocarbon radicals per molecule.

Ingredient A can be any polyorganosiloxane curable by aplatinum-catalyzed hydrosilylation reaction. The viscosity of ingredientA can range from a liquid to a high viscosity gum that will flow onlyunder pressure. Two of the ethylenically unsaturated hydrocarbonradicals presened in ingredient A are located at the terminal positionsof the molecule in the form of dimethylvinylsiloxy,methylphenylvinylsiloxy or dimethyl-1-hexenyl groups.

If the curable composition is a liquid or pasty material, the viscosityof ingredient A is preferably from 1 to 500 Pa.s. Polymers of this typeare well known and commercially available. In addition todiorganosiloxane and terminal triorganosiloxy groups ingredient A cancontain one or more monoorganosiloxane units per molecule, resulting inbranching of the polymer molecule. Polymers of this type are describedin U.S. Pat. No. 3,284,406 to Nelson, which issued on Nov. 8, 1966.

Alternatively, ingredient A can be a semi-solid polydiorganosiloxane,known in the art as a gum, exhibiting a viscosity of up to 1000 Pa.s orgreater at 25 degrees C. Curable compositions containing this type ofpolydiorganosiloxane are typically prepared by blending the ingredientsunder high shear using a two- or three roll rubber mill or dough-typemixer.

Surprisingly it has been found that the microencapsulatedhydrosilylation catalysts of this invention do not rupture or collapseunder the conditions used to process these high consistencyorganosiloxane compositions. The catalysts can therefore be incorporatedthis type of curable composition using conventional blending methods.

Ingredient A is cured by a hydrosilylation reaction between theethylenically unsaturated hydrocarbon radicals of this ingredient andsilicon-bonded hydrogen atoms of an organohydrogensiloxane, ingredientB. In a typical curable composition of this invention one or morepolydiorganosiloxanes containing an average of at least twoethylenically unsaturated hydrocarbon radicals per molecule react with arelatively low molecular weight, liquid organohydrogensiloxanecontaining an average of at least three silicon bonded hydrogen atomsper molecule.

The silicon-bonded hydrocarbon or substituted hydrocarbon radicals thatconstitute the organic groups bonded to the silicon atoms of ingredientsA and B are monovalent unsubstituted or substituted hydrocarbon radicalscontaining from 1 up to 20 or more carbon atoms. Halogen atoms arepreferred substituents. Preferably these hydrocarbon radicals are loweralkyl, phenyl or a perfluoroalkylethyl radical such as3,3,3-trifluoropropyl, this preference being based on the availabilityof the intermediates used to prepare ingredient A and B. Most preferablyat least a portion of the repeating units of ingredients A and B containsilicon bonded methyl radicals, and

Ingredient B can contain from as few as four silicon atoms per moleculeup to an average of 20 or more, and preferably exhibits a viscosity ofup to 10 Pa.s at 25 degrees C. Ingredient B contains repeating units ofthe formulae HSiO₁.5, RHSiO and/or R² HSiO₀.5. The molecules of thisingredient may also include one or more monoorganosiloxane,diorganosiloxane, triorganosiloxy and SiO_(4/2) units that do notcontain silicon bonded hydrogen atoms. In these formulae R is amonovalent hydrocarbon radical as defined in the preceding section ofthis specification.

Alternatively, ingredient B can be a cyclic compound containing at least4 organohydrogensiloxane units of the formula RHSiO or a compound of theformula HR₂ SiO(HRSiO)_(a) SiR₂ H, where a is at least 1.

Most preferably R is methyl and ingredient B is a linear trimethylsiloxyterminated polymethylhydrogensiloxane or adimethylsiloxane/methylhydrogensiloxane copolymer containing an averageof from 5 to about 50 repeating units per molecule of which from 30 to100 percent are methylhydrogensiloxane units.

The molecular weights of ingredients A and B together with the numberand distribution of the silicon-bonded hydrogen atoms and ethylenicallyunsaturated hydrocarbon radicals within these ingredients will determinethe location of crosslinks in the cured product, which can range from aglass-like resin to an elastomer to a gel.

The concentration of crosslinks per unit volume is often referred to asthe "crosslink density" and determines certain physical properties ofthe cured elastomer, particularly hardness, tensile strength andelongation. The particular combinations of polydiorganosiloxane(s) andcuring agent(s) yielding the desired combination of physical propertiescan readily be determined by routine experimentation with a knowledge ofthis invention.

The molar ratio of silicon bonded hydrogen atoms in theorganohydrogensiloxane to the vinyl or other ethylenically unsaturatedhydrocarbon radicals present in ingredient A is a major factor indetermining the properties of the elastomer or other cured materialobtained from the composition. Because of the difficulty oftenexperienced in achieving a complete reaction between all of the siliconbonded hydrogen atoms and all of the vinyl or other ethylenicallyunsaturated hydrocarbon radicals present in the reaction mixture, it isdesirable to have an stoichiometric excess of one of these species in acurable composition. A ratio of from 1.0 to 1.6 silicon bonded hydrogenatoms per vinyl or other ethylenically unsaturated hydrocarbon radicalhas been found to yield optimum combinations of physical properties. Thepreferred ratio for a given composition will be determined at least inpart by the average molecular weight of ingredient A and the type ofcuring agent.

The consistency of the present compositions can vary from a flowableliquid to a semi-solid paste to a high consistency gum that will flowonly under high shear. In addition to the aforementioned ingredients thecompositions can contain other additives including but not limited toreinforcing and non-reinforcing fillers, treating agents for thesefillers, pigments, processing aids, stabilizers and flame retardants. Itshould be understood that some of these additives will detract from theoptical transparency of the curable and cured organosiloxanecompositions containing the preferred platinum-containinghydrosilylation catalysts.

The amount of microencapsulated curing catalyst present in the curablecompositions of this invention is typically not restricted so long asthere is a sufficient amount to promote the reaction between ingredientsA and B. Because of the small particle size of the presentmicroencapsulated catalysts it is possible to use catalystconcentrations equivalent to as little as 1 part by weight or less,based on platinum-group metal, per million parts of curable compositionand still obtain a uniformly cured product.

For some applications it may be desirable to add one of the knownplatinum catalyst inhibitors to the present curable organosiloxanecompositions to alter the cure profile of the composition. It has beenfound that the presence of from about 100 to about 500 parts permillion, based on the weight of the curable composition, of an alkynoltype of platinum catalyst inhibitor will lengthen the induction periodprior to the initiation of the curing reaction, measured using a torquerheometer, while decreasing the time interval required to completecuring of the composition.

EXAMPLES

The following examples describe preferred embodiments of the presentmicroencapsulated curing catalysts methods for preparing these catalystsand one-part, storage stable curable organosiloxane compositionscontaining the microencapsulated curing catalysts. The examples shouldnot be interpreted as limiting the scope of the invention defined in theaccompanying claims. Unless otherwise specified all parts andpercentages are by weight and all viscosities were measured at 25degrees C. In the formulae Me represents the methyl radical and Phrepresent the phenyl radical.

Preparation of Propargyl Cyanoacetate

1.6 parts of propargyl alcohol and 1 part of cyanoacetic acid weredissolved in 4.7 parts of chloroform. 0.2 parts of a 98 percent byweight solution of aqueous sulfuric acid were added as a catalyst andthe resultant mixture was heated for 5.5 hours at a temperature of 61°C. The crude ester was washed with water. Volatile materials wereremoved using reduced pressure. The ester, represented the formulaN.tbd.CCH₂ C(O)OCH₂ C.tbd.CH, was isolated in 71 percent yield.

EXAMPLE 2 Reaction of Propargyl Cyanoacetate with (a) Cinnamaldehyde toform Propargyl 2-cyano-5-phenyl-2,4-pentanedienoate (PCPPD, formula 2a)and with (b) Naphthaldehyde

A Knoevenagel condensation was conducted by dissolving 15.5 g (0.126mol) of propargyl cyanoacetate in 40 cc of dioxane in an open flask withstirring. An equimolar amount (16.6 g) of cinnamaldehyde was then addedto the reaction mixture and the flask cooled in an ice-water mixture.0.4 cc of piperidine were then gradually over a period of about 10minutes as a reaction catalyst. A yellow solid precipitated in about 5minutes. The mixture was stirred briefly, then allowed to remain underambient conditions for 3.5 hours, at which time the solid was isolatedby filtration and washed with water. A 91% yield of crude productmelting at 133° C. was obtained. This material was recrystallized fromtoluene to yield a product melting at 138° C.

The infra-red and proton nuclear magnetic resonance spectra of therecrystallized compound exhibited maxima characteristic of the C.tbd.CH,C.tbd.N, C.tbd.C, C═O, --COO, CH₂ C.tbd.C and C═C--C═C groups, and wasconsistent with the expected product, PCPPD, corresponding to formula 2a

    PhCH═CHCH═C(CN)C(O)OCH.sub.2 C.tbd.CH

The corresponding condensation product using naphthaldehyde was preparedby replacing the cinnamaldehyde with an equimolar quantity ofnaphthaldehyde. This product melted at 116° C. and will be referred toas 2b. The formula for this product is the same as 2a, with theexception that Ph is replaced with the 1-naphthyl radical.

EXAMPLE 3 Hydrosilylation of PCPPD and the Corresponding NaphthaldehydeDerivative with 1,1,3,3-Tetramethyldisiloxane (TMDS)

A 16.7 percent solution of PCPPD in heated toluene was combined with anamount of a platinum hydrosilylation catalyst equivalent to 0.004 weightpercent platinum, based on the weight of toluene. The catalyst was areaction product of hexachloroplatinic acid andsym-tetramethyldivinyldisiloxane that has been diluted with a liquiddimethylvinylsiloxy terminated polydimethylsiloxane in an amountsufficient to achieve a platinum content of 4.2 weight percent. Theresultant solution was heated to a temperature of between 70° and 75° C.at which time an amount of 1,1,3,3-tetramethyldisiloxane (TMDS)equivalent to a molar ratio of silicon-bonded hydrogen atoms to C.tbd.Cradicals in the PCPPD of 1:1 was added dropwise to the reaction mixture.

Heating of the resultant reaction mixture was continued for anadditional four hours following completion of the TMDS addition, atwhich time the toluene was removed from the reaction mixture underreduced pressure to yield the desired PCPPD/TMDS reaction product as ayellow solid exhibiting a melting point of 78° C. The IR absorptionspectrum of this product exhibited maxima characteristic of the C.tbd.N,C═O, .tbd.Si--O--Si.tbd. and --CH═CH--CH═CH-- groups, which isconsistent with a compound corresponding to formula 1a.

    [PhCH═CHCH═C(CN)C(O)OCH.sub.2 CH═CH═SiMe.sub.2 ].sub.2 O(1a)

where Ph represents a phenyl radical and Me represents a methyl radical.

The naphthaldehyde derivative of the organosiloxane compoundcorresponding to formula 1a where PhCH═CH is replaced with the1-naphthyl radical was prepared using the same procedure used to prepareorganosiloxane compound 1a, but replacing propargyl ester 2a with anequimolar quantity of propargyl ester 2b.

Preparation of Microencapsulated Hydrosilylation Catalysts

A solution was prepared by dissolving (1) 1 part of organosiloxanecompound 1a prepared as described in the preceding section of thisexample, 1 part of organosiloxane compound 1b, or a mixture containing0.6 part of compound 1a and 0.4 part of compound 1b, (2) 0.6 part of acoordination complex of hexachloroplatinic acid andsym-tetramethyldivinyldisiloxane containing 4 weight percent of platinummetal in (3) 1 part of acetone.

Each of the resultant solutions was poured into a tray to form a 4mm-thick layer of liquid. The surface of the liquid was located 2 inches(5 cm.) below a 360 nm ultraviolet source. The energy content of theradiation produced by the source was 15 mw/cm2. The exposure time was 2hours, equivalent to a dosage of 108 joules/cm².

Examination of the resultant brownish-black waxy solid under amicroscope revealed the solid to be composed of microcapsules exhibitingan average diameter of 0.2 micrometer.

EXAMPLE 4 Evaluation of the Microencapsulated Catalyst of Example 3

Two catalyst master batch was prepared by blending one part of one ofthe three encapsulated platinum catalysts prepared as described inExample 3 on a three-roll mill with nine parts of a mixture prepared byblending to homogeneity (1) 100 parts of a dimethylvinylsiloxyterminated polydimethylsiloxane having a viscosity of about 2.1 Pa.s at25 degrees C., 2.9 parts of water, 9 parts of hexamethyldisilazane, and40 parts of a fume silica having a nominal surface area of 250 m² pergram.

A high consistency organosiloxane composition (I) was prepared byblending to homogeneity (1) 1.71 parts of a trimethylsiloxy- terminatedpolydiorganosiloxane having an average of five methylhydrogensiloxaneunits and three dimethylsiloxane units per molecule with asilicon-bonded hydrogen atom content in the range from about 0.7 to 0.8weight percent, and (2) 200 parts of a high consistency organosiloxanecomposition that had been prepared by blending the following ingredientsto homogeneity:

68 parts of a high consistency dimethylvinylsiloxy-terminatedpolydimethylsiloxane containing 0.142 mole percent ofmethylvinylsiloxane units and exhibiting a Williams plasticity of from1.4 to 1.7 mm.

26 parts of a fume silica having a nominal surface area of 250 m² pergram, and

1 part of a hydroxyl-terminated dimethylsiloxane/methylvinylsiloxanecopolymer containing about 10 weight percent of vinyl radicals and about16 weight percent of hydroxyl groups, and 5 parts of ahydroxyl-terminated polymethylphenylsiloxane having a viscosity of about0.5 Pa.s at 25° C. and a hydroxyl content of about 4.5 weight percent.

A curable composition of this invention was prepared by blending 0.48part or 0.24 part of each of the two catalyst master batches describedin the preceding section of this example was into separate batches ofthe high consistency composition I to form a curable composition of thepresent invention. Some of the compositions were also blended with 300ppm of 1-ethynyl-1-cyclohexanol (ETCH), a known platinum catalystinhibitor, to determine the effect of this additive on the storagestability and cure profile of the composition.

The cure profiles of the compositions were measured using a torquerheometer at a temperature of 170° or 190° C. The elapsed time requiredfor the torque to increase from the initial value and the elapsed timerequired for the torque to increase to 50 and 90 percent of the finalvalue (Tmax) are recorded in Table 1 as t2, t50 and t90, respectively.

A second portion of each of each curable composition was stored at 40°C. and the plasticity of each composition was measured 2 weeks and 4weeks after it had been prepared.

A third portion of each composition was cured for 10 minutes in ahydraulic press maintained at a temperature of 170° C. The samples wereallowed to cool for 16 hours prior to measurement of their compressionset.

The results of the storage stability and compression set determinationsare recorded in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    Organosiloxane                                                                Compound                                                                                                               Plasticity (mm)                      Type                                                                             Parts*                                                                            Cure Temp.                                                                           Tmax kN/M                                                                            t2 Min                                                                            t50 Min                                                                            t90 Min                                                                            Comp Set                                                                            Initial                                                                           2 wk                                                                              4 wk                                                                              ETCH                     __________________________________________________________________________    la 0.48                                                                              170    11.2   1.6 3.9  12.2 38    0.23                                                                              0.27                                                                              0.32                                                                              0                        la 0.48                                                                              190    12.3   1.2 3.0  9.8  27                                         lb 0.48                                                                              170    14.4   0.6 2.0  5.0  24    Cured in 4 wks. @25°                                                               0.                       la/lb                                                                            0.24                                                                              170    13.9   2.2 3.6  8.9  22    0.24                                                                              0.28                                                                              0.32                                                                              300                      la/lb                                                                            0.24                                                                              190    14.3   1.2 2.1  5.4  16        300                              la/lb                                                                            0.24                                                                              170    11.3   1.5 4.0  12.5 28    0.24                                                                              0.27                                                                              0.29                                                                              0                        la/lb                                                                            0.24                                                                              190    13.1   0.9 2.4  9.5  26                0                        la/lb                                                                            0.48                                                                              170    14.3   1.6 2.6  6.2  20    0.23                                                                              0.34                                                                              0.39                                                                              300                      la/lb                                                                            0.48                                                                              190    13.8   1.2 2.2  5.5  15        300                              __________________________________________________________________________     *parts of encapsulated catalyst by weight based on 201.71 parts of            composition minus catalyst                                               

That which is claimed is:
 1. A one-part, storage stable, heat curableorganosiloxane composition comprisingA. a liquid or gumpolyorganosiloxane containing an average of at east two ethylenicallyunsaturated hydrocarbon radicals per molecule; B. as a curing agent forsaid composition, an organohydrogensiloxane containing an average of atleast two silicon-bonded hydrogen atoms per molecule, and C. an amountof a microencapsulated hydrosilylation catalyst sufficient to promotecuring of said composition at temperatures above 100° C. or in thepresence of ultraviolet radiation having a wavelength in the range from200 to 260 nanometers,where the concentration of said curing agent issufficient to cure said composition, the sum of the average number ofethylenically unsaturated hydrocarbon radicals per molecule ofpolyorganosiloxane (A) and the average number of silicon-bonded hydrogenatoms in each molecule of said curing agent (B) is greater than 4, saidcatalyst is microencapsulated within a matrix or layer of at least onecrosslinked organosiloxane compound, the non-crosslinked organosiloxanecompound component used to make the crosslinked organosiloxane,comprises at least two units of the formula

    R.sup.2 (CH═CH).sub.n --CH═CR.sup.3 C(O)OCH.sub.2 CH═CH--SiR.sup.1.sub.a O.sub.(3-a/2)

where each R¹ is individually selected from the group consisting ofunsubstituted and substituted monovalent hydrocarbon radicals, R²represents an aryl, alkoxyaryl or alkaryl radical, R³ is --C.tbd.N or--C(O)OR⁴ where R⁴ is hydrogen or an unsubstituted monovalenthydrocarbon radical, a is 0, 1 or 2 and n is 0 or a positive integer,with the proviso that n can be 0 only when R² represents a naphthylradical, and said organosiloxane compound has been crosslinked byexposure to ultraviolet radiation in the wavelength range of from 300 to400 nm.
 2. A composition according to claim 1 whereR¹ is selected fromthe group consisting of alkyl radicals containing from 1 to 10 carbonatoms, cycloalkyl, aryl, alkaryl, aralkyl and substituted alkyl radicalswhere the substituent is a halogen atom; R² represents phenyl, o-, m-,or p-methoxyphenyl or naphthyl; n is 0, 1, or 2; said hydrosilylationcatalyst constitutes up to 5 percent of the combined weight of saidcatalyst and said crosslinked organosiloxane compound; the averageparticle diameter of the microencapsulated hydrosilylation catalyst isless than 3 micrometers; the ethylenically unsaturated hydrocarbonradicals present in said polyorganosiloxane are vinyl or 5-hexenyl; theremaining silicon bonded hydrocarbon radicals present in saidpolyorganosiloxane and said organohydrogensiloxane are selected from thegroup consisting of alkyl containing from 1 to 4 carbon atoms, phenyl,and 3,3,3-trifluoropropyl; and said hydrosilylation catalyst is acompound of a platinum-group metal.
 3. A composition according to claim2 where R¹ is methyl, phenyl, or 3,3,3-trifluoropropyl; R³ is a cyanogroup; n is 0 and R² is naphthyl or n is 1 and R² is phenyl; a is 1 or2; at least 50 percent of said microencapsulated catalyst exhibits aparticle diameter below one micrometer; said hydrosilylation catalyst isa coordinated platinum compound prepared by reacting chloroplatinic acidwith an ethylenically unsaturated organosilicon compound; saidpolyorganosiloxane is a gum exhibiting a viscosity greater than 1000Pa.s at 25° C.; and the viscosity of said organohydrogensiloxane is upto 10 Pa.s at 25° C.
 4. A composition according to claim 3 where saidcatalyst is microencapsulated within a layer consisting essentially of afirst crosslinked organosiloxane compound wherein R¹ is methyl; R² isphenyl, R³ is a cyano group, a is 2 and n is 1; and a second crosslinkedorganosiloxane compound wherein R¹ is methyl, R² is naphthyl; R³ is acyano group; a is 2; n is 0; and said first organosiloxane compoundconstitutes from 50 to 95 percent of the combined weight of said firstand second organosiloxane compounds.
 5. A composition according to claim3 where said composition contains an amount of an platinum catalystinhibitor sufficient to alter the curing profile of said composition. 6.A composition according to claim 5 where said inhibitor is an alkynol.